usually,when you buy a harness there are two main power wires and a ground. On mine there was a yellow wire which was the acc wire (hot at all times) that comes on when you turn the key but not start the engine and run down the battery. then there was a red wire that would turn on when the engine was on. Then of course black was the ground.
As far the blue wire, on my setup, it was a "remote wire." am not a 100% on this but speculating from the name, it turns the volume up and down on the subs, but i don't know for sure. I assume your "p.con." wire is the same.